1. Field of the Invention
The present invention is related to opto-electronic circuits and more particularly to temperature compensated opto-electronic circuits containing both an emitter and a detector.
2. Description of the Prior Art
The term "opto-electronics" typically refers to a general area of technology related to devices which emit, detect, or emit and detect electromagnetic (e/m) radiation in the visible, infrared, or ultraviolet spectral regions of the frequency spectrum.
FIG. 8 illustrates a simplified opto-electronic circuit 800 used, for example, in an opto-mechanical mouse. The opto-electronic circuit 800 includes a light emitting diode (LED) 810 for generating e/m radiation, a phototransistor 820 for receiving the e/m radiation, a comparator 840 for comparing a detector signal generated by the phototransistor 820 with a reference signal, and a power supply 850. The anode of the LED 810 is connected to the power supply 850 and the cathode is connected to ground through a load resistor 815. The collector of the phototransistor 820 is connected to the power supply 850. The emitter of the phototransistor 820 is connected both to the non-inverting input of the comparator 840 and to ground through a load resistor 825. Finally, a reference voltage is supplied to the inverting input of comparator 840 through a voltage divider 860 comprised of a first resistor 861 and a second resistor 862.
In operation, displacement of the opto-mechanical mouse relative to a fixed reference results in a rotation of an encoder 830 which is located between the LED 810 and the phototransistor 820. As the encoder 830 rotates, e/m radiation from the LED 810 passes through a plurality of openings formed on the encoder 830 and strikes the phototransistor 820 as a series of e/m pulses. Each e/m pulse causes the phototransistor 820 to turn on, thereby transmitting a high detector signal to the non-inverting input of the comparator 840. The frequency of the detector signal is directly proportional to the rotating speed of the encoder 830. The comparator 840 generates an OUTPUT signal having an amplitude which is determined by a difference between the detector signal and the reference voltage. The OUTPUT signal is typically used to control the position of a cursor on a video terminal screen.
A problem with the above-described opto-electronic circuit is that the detector signal varies inversely with ambient temperature. That is, as the temperature of the circuit decreases, the detector signal (voltage) applied to the non-inverting input of comparator 840 increases. Conversely, as the ambient temperature increases, the detector signal decreases. On the other hand, similar ambient temperature changes have little or no effect on the reference voltage. As a result, as the detector signal applied to the non-inverting input terminal of the comparator 840 varies with ambient temperature changes, the duty cycle of the comparator 840 is correspondingly changed.
FIG. 9 shows a typical temperature/current diagram associated with a silicon phototransistor in a prior art circuit. As indicated, the collector current decreases from 1.05 to 0.91 milliamps over a temperature change from -5.degree. C. to 55.degree. C., representing an approximate 14% variance.
FIGS. 10(a) to 10(d) illustrate the effect of temperature on the duty cycle of the comparator 840.
FIG. 10(a) shows a reference voltage and an ideal detector signal at a constant temperature. The 1 volt reference voltage 1010, represented as a horizontal line, is applied to the inverting input terminal of the comparator 840. The ideal detector signal 1020 generated by a constant rotation of the encoder 830 is shown as a solid sinusoidal wave. As shown, the ideal detector signal 1020 has a peak to peak amplitude of 2 volts, and swings about the 1 volt reference voltage.
FIG. 10(b) shows a square wave OUTPUT signal 1030 generated by the comparator 840 in response to the ideal detector signal 1020 and the reference voltage 1010 of FIG. 10(a). Because the peaks of the detector signal 1020 are symmetric about the reference voltage 1010, the OUTPUT signal 1030 generated by the comparator 840 has an ideal duty cycle of 50%.
FIGS. 10(c) and 10(d) show an effect of temperature variations on the OUTPUT signal generated by the comparator 840. Referring to FIG. 10(c), the reference voltage 1010 remains constant at 1 volt for all temperatures because the resistors 860 and 865 (FIG. 8) which produce the reference signal 1010 are not significantly effected by temperature variations. As the temperature decreases, a detector signal 1020(A) is generated by the phototransistor 820 which has a base (average) voltage shifted upward to 1.5 volts, and cycles between 0.5 and 2.5 volts.
FIG. 10(d) shows the duty cycle of the OUTPUT signal 1030(A) generated by the comparator 840 in response to the detector signal 1020(A). Because the detector signal 1020(A) is greater than the reference signal 1010 for an increased percentage of time due to the upward shift of the base voltage level of detector signal 1020(A), the duty cycle of the comparator 840 is increased correspondingly.
One prior art method of compensating for changes in the detector signal due to temperature variations is to replace one of the first resistor 861 or second resistor 862 of the voltage regulator 860 with a thermistor. The thermistor is used to adjust the reference signal to correspond with increases and decreases in the detector signal due to temperature variations, thereby normalizing the duty cycle of the comparator 840. However, this solution increased the cost of the circuit due to the cost of the thermistor.